ITU-T Recommendation H.264|ISO/IEC 14496-10 Advanced Video Coding Amendment 1: Fidelity Range Extensions (MPEG4-AVC FRExt, Joint Video Team (JVT) draft JVT-L050d4) is a significant proposed enhancement of the original ITU-T Recommendation H.264 & ISO/IEC 14496-10 “Advanced Video Coding” video coding standard. One of the most significant extensions, highlighted in the title of the amendment, is the increased video fidelity that is enabled with an increased range of coded transform coefficients. The FRExt 8-bit video “High Profile” extension is rapidly being provisionally adopted by DVB, DVD-Forum, Blu-ray, ATSC, and other such application standards bodies for such markets as HD-DVD players/recorders/content-distribution, set-top boxes, personal video recorders (PVRs), etc.
One significant drawback of the draft standard is that a bit stream containing 17-bit quantized coefficients is compliant with the standard when 8-bit video is represented (e.g., with the High Profile (HP) extension). The disadvantage is that HP compliant decoders (and in particular HP decoders implemented on 16-bit DSPs) must be capable of processing 17-bit coefficients. The ability to process 17-bit coefficients is a significant burden for HP compliant decoders, particularly for architectures relying on 16-bit components (e.g., acceleration on Intel® platforms using MMX™ instructions) for consumer applications. In general, the vast majority of consumer applications use 8-bit video.
Decoders must be designed to operate with worst-case bit streams to be considered standard compliant. Therefore, if (with a particular architecture) being fully compliant is not possible at reasonable cost, many decoders may choose to be partially compliant (e.g., be able to compliantly decode 8-bit video streams containing only 16-bit coefficients), or be able to display ‘reasonable’ although non-compliant output for video streams containing 17-bit coefficients (e.g., (i) drop frames if 17-bit coefficients require too many processing cycles, (ii) approximate the output by approximating 17-bit coefficients with 16-bit coefficients, (iii) output corrupted and/or drifting output when 17-bit coefficients are encountered, (iv) be able to compliantly decode 17-bit coefficients for luma and/or chroma only and (v) variations on the above. However, non-compliant decoders can suffer various market acceptance and licensing risks that are not present for compliant decoders. Decoders capable of decoding worst-case bit streams containing 17-bit coefficients can suffer substantial cost premiums over decoding hardware and software that cannot compliantly decode such bitstreams.
The JVT-L050d4 draft FRExt specification method for imposing limitations on bit streams cannot prevent compliant streams from containing 17-bit coefficients for 8-bit video. The FRExt standard specified range limits on the outputs of inverse transform butterfly stages and coefficient scaling operations (as used by both the draft FRExt standard and the original non-amended standard) are insufficient to indirectly limit all coefficients to 16-bits for 8-bit video data in High Profile format.
It would be desirable to have a method for specification of a quantized coefficient limit.